1. Field of the Invention
This invention relates to a new process for the production of aqueous dispersions of polyurethane polyureas in which polyurethane polyureas containing free isocyanate groups are first prepared in organic solution using selected starting materials in selected quantitative ratios and the resulting solutions are subsequently dispersed in water, to the dispersions obtainable by this process and to their use as coating compositions for hard, inflexible substrates.
2. Description of the Prior Art
Aqueous dispersions of polyurethanes are being used to an increasing extent for the production of lacquer coatings having a high property level. In addition to the lacquering of wood, they are being increasingly used for external applications where high weather resistance and durability are essential. Plastic components for automobiles are already being coated with aqueous air-drying lacquers while their application as a metallic base coat to automobile bodies is a major potential field of application which is attracting considerable interest simply by virtue of the possibility of reducing solvent emission.
Aqueous polyurethane dispersions wherein the solids are based on difunctional units comprising components containing carboxyl groups have been held in readiness for these applications for some years. The carboxylic acid groups are converted with tertiary amines into salt groups which guarantee the dispersibility of the solids and have the particular advantage of gradually losing their ionic character in the final lacquer film through evaporation of the tertiary amine (cf. for example U.S. Pat. No. 4,066,591). This provides the lacquer films with outstanding resistance to water. Unfortunately, dispersions of the type in question are attended by two significant disadvantages, namely,
their solvent and weather resistance are poor, PA1 their stability in storage is limited, particularly when the dispersion is intended to be substantially neutral, i.e. to have a pH of about 7. PA1 (a) NCO-prepolymers based on (i) organic polyhydroxyl compounds and, optionally, organic monohydroxyl compounds and (ii) excess quantities of organic polyisocyanates and, optionally monoisocyanates with PA1 (b) organic polyamines PA1 R is a difunctional residue obtained by removal of the isocyanate groups from a diisocyanate of the above-mentioned type corresponding to the formula R(NCO).sub.n wherein n=2, PA1 R' is a monofunctional hydrocarbon radical containing from 1 to 12 carbon atoms, preferably an unsubstituted C.sub.1 -C.sub.4 alkyl group, PA1 X is a polyalkylene oxide chain containing about 5 to 90, preferably about 20 to 70 chain members, wherein at least about 40%, preferably at least about 65% are ethylene oxide units and which, in addition to ethylene oxide units, may also contain propylene oxide, butylene oxide or styrene oxide units, preferably propylene oxide units, and PA1 Y is preferably oxygen or even -NR"-, where R" corresponds in its definition to R.
Efforts to obviate the first disadvantage have led to processes in which polyfunctional units are used, generally in the chain extension of the NCO-prepolymers on which the solids are based (cf. for example U.S. Pat. Nos. 4,203,883 and 3,870,684). However, these dispersions are attended by the disadvantage of inadequate stability in storage. An improvement in this respect was achieved by the incorporation of nonionic hydrophilic groups which partly replace the carboxylate groups of the polyurethane (U.S. Pat. No. 4,408,008). However, the production of aqueous dispersions of branched polyurethanes has previously involved a number of process steps with unfavorable effects.
1. For thorough dispersion in water, the polymers to be dispersed in water should have only a limited viscosity. This precludes in particular the use of more than negligible quantities of chain-branching components. Although the polymers to be dispersed in water may be diluted with high-boiling polar solvents to overcome this viscosity limit, the result in many applications is a significant delay in the drying of the lacquer coatings. PA0 2. Because it competes with the isocyanate/water reaction and particularly because it takes place in a heterogeneous system, chain extension or chain branching in aqueous phase, i.e. addition of the diamines and polyamines to the prepolymer already dispersed in water, has the effect that a residue of unreacted amino groups remains behind, increasing the pH-value of the resulting dispersion to distinctly above 7 and normally to between 8 and 9.0. However, aqueous dispersions having a pH-value in this range are unsuitable for the production of high-quality, aluminium-pigmented metallic lacquer coatings because aluminum is significantly attacked by aqueous systems in this pH-range with formation of aluminum oxide causing the lacquer coating to assume a dull appearance. PA0 1. The process should be workable without the use of highly polar, high-boiling auxiliary solvents. PA0 2. The resulting dispersions should show high stability in storage. PA0 3. The resulting dispersions should be substantially neutral, i.e. should have a pH value of approximately 7. PA0 4. The dispersed polyurethane polyurea should have a certain degree of branching so that the hardness and solvent resistance of the coatings produced from them meet the stringent requirements imposed on these two properties in practice. PA0 (a) the NCO-prepolymers used are NCO-prepolymers having an average NCO-functionality of 2 to 2.3 and an average molecular weight of about 700 to 5000 PA0 (b) the organic polyamines used are polyamines containing primary and/or secondary amino groups or mixtures thereof having an(average)amine functionality of 2.05 to 3 and an(average)molecular weight of 60 to about 2000
Although it is possible to prepare aqueous polyurethane dispersions by the so-called "acetone process," i.e. to synthesize the chain-extended high molecular weight polyurethane in solution in acetone and then to convert the dissolved polyurethane into an aqueous dispersion by mixing the solution with water and distilling off the solvent, known processes for the preparation of aqueous dispersions of branched or crosslinked polyurethanes have never made use of this possibility because it has previously hitherto been regarded as an established fact that the synthesis of a polyurethane in weakly polar, low-boiling solvents, such as acetone, is only possible when difunctional units are almost exclusively used.
Accordingly, an object of the present invention is to provide a new process for the production of aqueous dispersions of amine-extended polyurethanes, i.e. polyurethane polyureas, which in particular combines the following advantages.
This object may be achieved by the process according to the invention which is described in detail hereafter.